Endoscopic instruments of this type are frequently used in medicine and serve for the observation, examination and treatment of body cavities either using a natural body orifice or an artificially created incision channel. All endoscopic instruments have in common an optical observation system which is closed off at the distal shaft end by a window. This window is connected tightly and firmly with the shaft. If this is not a video endoscope, an eyepiece is provided, as a rule, at the proximal end of the instrument. Furthermore, an illuminating system can be associated with the optical observation system; then a fiber optic light guide connector is additionally provided in the proximal region of the shaft.
Usually, the closure windows of such endoscopic instruments consist of glass. Although the concern with regard to the optical components lying inside the shaft involves almost exclusively their imaging characteristics, further requirements are placed on a closure window, in particular of a mechanical and material nature. On the one hand, as great a hardness as possible is desired for the closure window, in order to largely rule out impairment by possible scratches. On the other hand, in addition to this abrasive resistance, care must also be taken that sterilization is possible without loss of optical quality. Thus, for example, with an established sterilization cycle, the endoscopic instrument is exposed for a predetermined period to H.sub.2 O saturated superheated steam at a temperature of 134.degree. C.
Only a few types of glass are suitable for such sterilization cycles, and even these only with limitations. The reason for this is that, as is known, at high temperatures certain water-soluble substances are washed out from the surface of the glass, which leads to loss of optical quality. Also corrosion phenomena on the exterior glass surfaces are brought about through repeated sterilization, which are due essentially to the reciprocal effect of the metallic oxides contained in the glass with the water vapor. Substances indeed exist which are similar to glass, with a partially amorphous structure, which can cope with this stress, but these cannot be used for optical purposes owing to intense inhomogeneities.
Better results are achieved, on the other hand, with monocrystalline sapphire as a material for the closure window. The use of such a material is described for example in DE 37 40 416 A1. A crystalline sapphire has a high level of surface hardness and can be readily exposed to the sterilization cycles described above, but owing to its hexagonal crystal lattice, this material displays an anisotropic behavior for various optical and physical characteristics which are particularly undesirable for use in endoscope optical systems. Here, especially, the problem of double refraction is to be stressed.
The refractive indices of this material have between the axis parallel to the crystal axis and the vertical a difference on the order of 9.times.10.sup.-3. In light beams which enter the crystal obliquely to the optical axis of the crystal, this double refraction causes a splitting into two polarized partial beams which extend vertically with respect to each other. It is therefore an indispensable requirement in the production and incorporation of closure windows of sapphire to pay precise attention to the orientation of the crystal. This is very costly in terms of manufacturing technology.
However, even with attention being paid to the crystal orientation and the prescribed production and incorporation of such a closure window, it is not possible, apart from the parallel light passage, to completely circumvent the effect of double refraction. In fact, owing to the necessary large field of vision in connection with small system diameters in endoscopic optics, very large apertures are necessary. This leads to the beams passing through the window in a large angle range, whereby differences in refractive index also make themselves apparent in practice. This results in a non-correctable loss of image quality.